Mechanism of Action

Histamine (one of the key mediators released from mast cells and basophils), plays a major role in the pathophysiology of allergic diseases, including rhinitis, urticaria, asthma and anaphylaxis. Histamine exerts its effects through its interaction with one of four distinct receptors (H1, H2, H3, H4). In allergic disease, it is the H1 antihistamines which are of primary benefit, although H2 antihistamines may also play a therapeutic role. H1 antihistamines remain first-line medications for the treatment of allergic rhinoconjunctivitis and urticaria. Second-generation antihistamines are preferred to their predecessors because of better benefit-to-risk ratios. The newer antihistamines are not only more potent, but also have anti-allergic and antiinflammatory properties. Although they are more expensive than the traditional antihistamines, the cost is substantially offset by their superior efficacy and safety profile when used in recommended dosages. Correspondence: Prof CasMotala, [email protected] Table I. Common H1receptor antagonists First generation Second generation* Hydroxyzine Cetirizine Diphenhydramine Loratadine Chlorpheniramine Desloratadine Promethazine Fexofenadine